In modern circuit designs, the frequency selective circuit, such as the low pass filter (LPF), is an indispensable block for filtering signals. For example, an inphase/quadrature modulator (IQM) may cooperate with a simple LPF cascaded by passive RC (resistor, capacitor) filters to generate signals to be transmitted. Once the frequency of unwanted signal is close to the desired signal, such LPF cannot provide enough rejections because of the low Q.
For getting better rejections, the designer needs to cascade more stages of RC filters, which occupied larger area, generates lager output noise and may corrupt the desired signal.
Therefore, there is a need of a frequency selective circuit capable of performing high Q filtering and reducing the output noise.